Convergence means for a plural beam color picture tube

ABSTRACT

In a color cathode-ray tube of the plural-beam type wherein the beams originate on a horizontal or vertical straight line and are directed into fields produced by a horizontal-vertical electromagnetic deflection yoke at predetermined incident angles to each other so as to converge at a color screen, magnetic shielding means, for example, secured to electron beam convergence plates, are disposed for selectively shielding at least one of the beams from the leakage flux produced by the horizontal-vertical electromagnetic deflection yoke in deflecting the beams in the direction of the line of origination thereof, whereby such leakage flux acts selectively on only the unshielded beam or beams for correcting a deviation between the positions of rasters on the color screen produced by the plural beams.

This invention relates generally to color picture tubes of theplural-beam type, and particularly to tubes of that type in which theplural beams are made to converge at a common point on a beam-selectinggrid or mask associated with the color phosphor screen at which thebeams are focused.

In single-gun, plural-beam color picture tubes of the described type,for example, as specifically disclosed in U.S. Pat. No. 3,448,316,granted June 3, 1969, and having a common assignee herewith, threelaterally spaced electron beams are emitted or originated by abeam-generating or cathode assembly and directed in a commonsubstantially horizontal or vertical plane with the central beamcoinciding with the optical axis of the single electron lens and the twoouter beams being converged to cross the central beam at the opticalcenter of the lens and thus emerge from the latter along paths that aredivergent from the optical axis. Arranged along such divergent paths arepairs of convergence deflecting plates having voltages appliedthereacross to deflect the divergent beams substantially in the plane oforigination thereof for causing all beams to converge at a point on theapertured beam-selecting grid or shadow mask associated with the colorscreen. After passing between the convergence deflecting plates, thebeams are acted upon by the magnetic fields resulting from theapplication of horizontal and vertical sweep signals to thecorresponding coils of a deflection yoke, whereby the beams are made toscan the screen in the desired raster. It will be apparent that, whenthe three beams are deflected by the yoke from a point of convergence atthe center of the screen, as during scanning, the distances that suchbeams travel through the magnetic fields of the deflection yoke arerelatively varied and spherical aberration results, that is, the beamsundergo different degrees of deflection resulting in misconvergence ofthe beams, particularly when the latter are directed at corner portionsof the screen.

Although certain aspects of the above-described misconvergence can becorrected by suitably shaping and dimensioning the horizontal andvertical deflection coils, for example, as hereinafter described indetail, there remains a horizontal deviation of the raster of thecentral beam with respect to the rasters of the other two beams,particularly at the opposite side portions of the screen in the case ofthe beams originating in a horizontal plane.

Accordingly, it is an object of this invention to avoid theabove-mentioned horizontal deviation of the rasters from each other,particularly at the opposite side portions of the screen of a colorpicture tube of the described type, without resorting to complex dynamicconvergence devices for that purpose.

Another object is to achieve the desired registration of the rasters byshielding one or more of the electron beams from a correction fieldwhich is applied to the remainder of the electron beams.

A further object is to achieve registration of the rasters by shieldingthe side beams from flux leakage from the horizontal deflection magneticfield produced by the deflection yoke, while permitting such leakageflux to act in a space through which one, for example, the central,electron beams passes, whereby to impart an additional horizontaldeflection to the central beam as compared with the side beams.

In accordance with an aspect of this invention, a plural-beam colorpicture tube, as described, is provided with magnetic shielding membersdisposed at outer sides of the paths of the side beams, preferably atthe exit for the latter between the convergence deflecting plates, andeach of the shielding members has a straight portion extending at rightangles to the plane in which the beams originate and end portionsprovided on the ends of the straight portion and extending inwardly atsubstantial angles to the latter to act as magnetic shields so thatleakage flux from the deflection yoke coil intended to deflect the beamsat right angles to said plane is substantially prevented from acting onthe side beams while being free to act on the central beam and thus fromthe correction field applied to the latter.

The above, and other objects, features and advantages of this invention,will become apparent from the following detailed description of anillustrative embodiment which is to be read in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic horizontal cross-sectional view showing aplural-beam, single-electron-gun-type color cathode-ray tube of the typeto which the present invention can be applied;

FIGS. 2A and 2B are front and side views showing the mechanicalarrangement of deflection yoke means applicable to the color cathode-raytube of FIG. 1;

FIG. 3 is a diagrammatic view illustrating the manner in which the beamsare deflected and converged, and showing the deviations thereof fromcorrect convergence to be corrected by this invention;

FIG. 4 is a horizontal cross-sectional view of the main portion of thecolor tube of FIG. 1, but shown with an embodiment of the presentinvention applied thereto;

FIG. 5 is a transverse sectional view taken along line X--X of the tubeof FIG. 4; and

FIG. 6 is a diagrammatic view showing the magnetic field distributionoccurring in the embodiment of the invention illustrated by FIGS. 4 and5.

In order to provide a better understanding of the present invention, athree-beam single-gun-type color cathode-ray tube of the type disclosedin detail in U.S. Pat. No. 3,448,316, and to which this invention may beapplied is generally described below.

Referring to the drawings, and initially to FIG. 1 thereof, it will beseen that the single-gun, plural-beam color picture tube 10 there shownmay comprise a glass envelope (shown in dotted lines) having a neck, anda cone extending from the neck to a color screen S provided with theusual arrays of color phosphors S_(R), S_(G) and S_(B), and with anapertured beam-selecting grid or shadow mask G_(P). Disposed within theneck is a single electron gun having cathodes K_(R), K_(G) and K_(B),each of which is constituted by a beam-generating source with therespective beam-generating surfaces thereof disposed as shown in a planewhich is substantially perpendicular to the axis of the electron gun. Inthe embodiment shown, the beam-generating surfaces are arranged in astraight line so that the respective beam B_(R), B_(G) and B_(B) emittedtherefrom are directed in a substantially horizontal plane containingthe axis of the gun, with the central beam B_(G) being coincident withsuch axis. A first grid G₁ is spaced from the beam-generating surfacesof cathodes k_(R), K_(G) and K_(B) and has apertures g_(1R), g_(1G) andg_(1B) formed therein in alignment with the respective cathodebeam-generating surfaces. A common grid G₂ is spaced from the first gridG₁ and has apertures g_(2R), g_(2G) and g_(2B) formed therein inalignment with the respective apertures of the first grid G₁.Successively arranged in the axial direction away from the common gridG₂ are open-ended, tubular grids or electrodes G₃, G₄ and G₅,respectively, with cathodes K_(R) K_(G) and K_(B), grids G₁ and G₂, andelectrodes G₃, G₄ and G₅ being maintained in the depicted assembledpositions thereof, by suitable, nonillustrated support means of aninsulating material.

For operation of the electron gun of FIG. 1, appropriate voltages areapplied to grids G₁ and G₂ and to electrodes G₃, G₄ and G₅, and, as aresult, an electron lens field will be established between grid G₂ andthe electrode G₃ to form an auxiliary lens L' as indicated in dashedlines, and an electron lens field will be established around the axis ofthe electrode G₄, by the electrodes G₃, G₄ and G₅, to form a main lensL, again as indicated in dashed lines.

Further included in the electron gun of FIG. 1, are electron beamconvergence deflecting means F which comprise shielding plates P and P'disposed in the depicted spaced, relationship at opposite sides of thegun axis, and axially extending, deflector plates Q and Q' which aredisposed, as shown, in outwardly spaced, opposed relationship inshielding plates P and P', respectively. Although depicted assubstantially straight, it is to be understood that the deflector platesQ and Q' may, alternatively, be somewhat curved or outwardly bowed, asis well known in the art.

The shielding plates P and P' are equally charged by a voltage V_(P) anddisposed so that the central electron beam B_(G) will pass substantiallyundeflected between the shielding plates P and P', while a voltage V_(Q)is applied to the deflector plates Q and Q' so that the latter havenegative charges with respect to the plates P and P', whereby electronbeams B_(B) and B_(R) will be convergently deflected as shown by therespective passages thereof between the plates P and Q and the plates P'and Q'.

In operation, the respective electron beams B_(R), B_(G) and B_(B) whichemanate from the beam-generating surfaces of cathodes K_(R), K_(G) andK_(B) will pass through the respective apertures of grid G₁ to beintensity modulated with what may be termed "red," "green" and "blue"intensity modulation signals applied between the cathodes and the firstgrid G₁. The respective electron beams will then pass through the commonauxiliary lens L' to cross each other at the center of the main lens Land emerge from the latter with beams B_(R) and B_(B) diverging frombeam B_(G). Thereafter, the central electron beam B_(G) will passsubstantially undeflected between shielding plates P and P' since thelatter are at the same potential. When electron beam B_(B) passesbetween plates P' and Q' and electron beam B_(R) passes between plates Pand Q, they will converge as a result of the convergence deflectingvoltage applied therebetween. The system of FIG. 1 is arranged so thatthe electron beams B_(B), B_(G) and B_(R) will desirably converge orcross each other at a common spot centered in an aperture betweenadjacent grid wires g_(P) of the beam-selecting grid or mask G_(P) so asto diverge therefrom to strike the respective color phosphors of acorresponding array on screen S.

Thus, to summarize the operation of the depicted color picture tube ofFIG. 1, the respective electron beams B_(B), B_(G) and B_(R) areintended to be converged at screen grid G_(P) and to diverge therefromin such manner that electron beam B_(B) will strike the "blue" phosphorS_(B), electron beam B_(G) will strike the "green" phosphor S_(G) andelectron beam B_(R) will strike the "red" phosphor S_(R) of the array orset corresponding to the grid aperture at which the beams converge.

Electron beam scanning of the face of the color phosphor 1968, iseffected in a conventional manner, for example, by horizontal andvertical electromagnetic deflection means 20 indicated in broken lines.Deflection means 20 may be constructed as a deflection yoke havinghorizontal and vertical deflection coils wound in a saddlelike ortoroidal form. By selecting the winding angle and position of thedeflection coils so that a pin-cushion shaped horizontal deflectionfield is produced by the horizontal deflection coil and a barrel-shapedvertical deflection field is produced by the vertical deflection coil,it is possible to effect dynamic convergence by the magnetic fieldsproduced by the deflection yoke, as described in detail in U.S. Pat.application Ser. No. 753,694, filed Aug. 19, 1968, which is now U.S.Pat. No. 3,500,114, granted Mar. 10, 1970, and having a common assigneeherewith.

An example of a deflection yoke means capable of producing apin-cushion-shaped deflection field, and a barrel-shaped verticaldeflection field is shown at 20 in FIGS. 2A and 2B. In FIGS. 2A and 2B,a deflection yoke 21 is provided on a yoke-supporting annular member 24which is enlarged at its front end in the form of a funnel. A pair ofvertical deflection coils V_(a) and V_(b) are symmetrically wound in atoroidal form on deflection yoke 21 with respect to horizontal planeH--H passing through the axis of the yoke. These vertical deflectioncoils are connected, for example, in series with each other. In order toproduce a barrel-shaped vertical deflection field, the winding angleφ_(V), at which the vertical deflection coils V_(a) and V_(b) are woundon the yoke 21, is selected to be greater than that which would producea rectangular vertical deflection field. In the case of a uniformlydistributed winding, the winding angle is set between 120° and 160° .

A pair of saddle-shaped horizontal deflection coils H_(a) and H_(b)extend within annular member 24 and are symmetrically located withrespect to horizontal plane H--H. These horizontal deflection coils areconnected, for example, in series with each other. In order to produce apin-cushion-shaped field, the left-hand side effective coil portion 22Lof horizontal deflection coil H_(a), and the left-hand side effectivecoil portion 23L of horizontal deflection coil H_(b) are disposed incontact, or in closely spaced relationship with each other. Similarly,the right-hand side effective coil portion 22R of coil H_(a) and theright-hand side effective coil portion 23R of coil H_(b) are disposed incontact, or closely spaced relationship with each other. The windingangles φ_(H) of coil portions 22L and 23L, and of coil portions 22R and23R are selected to be between 120° and 130°. The front portions ofcoils H_(a) and H_(b) adjacent the wide end of support 24 areconstructed in the form of a winding represented by the nth power of thecosine, or cos.sub. n, where n is a positive number between 2 and 7. Therear portions of coils H_(a) and H_(b) are constructed in the form of awinding represented by the mth power of the cosine, or cos^(m), where mis a positive number between 1 and 3.

With the above arrangement, even if horizontal and vertical convergencevoltage and current generating circuits, and vertical convergence meansare omitted, dynamic convergence in both the horizontal and verticaldirections can be effected with respect to the three beams B_(R), B_(B)and B_(G) when these beams are made to scan screen S.

The three beams B_(R), B_(G) and B_(B), when being deflectedhorizontally and vertically are located in a common plane which isinclined with respect to the horizontal plane H--H through an anglecorresponding substantially to the angle of vertical deflection, as thebeams are always arranged on a substantially horizontal line. However,the three beams in the common plane enter into the deflection yoke means20 at difference incident angles due to convergence means F. Thus, ifthe deflection yoke means of FIGS. 2A and 2B is not employed, there is atendency that the three beams will cross each other at a position whichis short of the beam-selecting grid or mask when these beams aredirected to the left or right-hand side portion thereof. However, byusing the deflection yoke means of FIGS. 2A and 2B, beam B_(B) isdeflected from its deflection center position across a field portion ofminimum strength corresponding to the position of the deflection centerfor beam B_(G) and beam B_(R) is deflected from its deflection centerposition through a relatively strong portion of the pin-cushion-typefield. Thus, the three beams can be made up to converge accurately witheach other at the beam-selecting grid or mask. For vertical deflections,if the deflection yoke means of FIGS. 2A and 2B is not used, the threebeams tend to cross each other short of the screen at the opposite sidesas in the horizontal deflection. By using the deflection yoke means ofFIGS. 2A and 2B, however, the three beams are subjected to substantiallythe same component of a barrel-type field so as to converge with eachother at the grid or mask G_(p), since they are not vertically spacedapart from each other.

Thus by winding the horizontal deflection coil in a saddlelike form, andby winding the vertical deflection coil in a toroidal form correspondingwith the curved surface of screen S, vertical dynamic convergence can beeffected without using any vertical dynamic convergence voltage andcurrent generating circuits. The configurations of the pin-cushion andbarrel magnetic fields can be determined by winding angle φ_(V) ofvertical deflection coils V_(a) and V_(b) and their positions on yoke21, and winding angle φ_(H) of horizontal deflection coils H_(a) andH_(b) and their positions within support 24. Thus, effective convergencecan be achieved without providing dynamic convergence means as normallyrequired by convention color cathode-ray tubes, or on the other hand,more effective convergence can be obtained by using such dynamicconvergence means at the same time.

By winding the horizontal deflection coils in a saddlelike form, it ispossible to easily change the configurations of the portion of thehorizontal deflection field on the screen side and that on the electrongun side so that, for example, one of the field portions can be of thebarrel type, while the other field portion is of the pin-cushion type,while the remainder of the horizontal deflection field is either apin-cushion-type or a barrel-type field. This would become difficult toachieve if the horizontal deflection coils were wound in toroidal form.

Usually rasters appearing on the screen tend to be subjected topin-cushion distortion due to the configuration of the screen in spiteof setting the focusing adjustment to achieve the best possible beamfocusing. However, by affording a pin-cushion shape to the horizontaldeflection field portion at the screen side and affording a barrel shapeto the horizontal deflection field on the electron gun side, inaccordance with the saddle-like configuration of the horizontaldeflection coils it is possible to easily correct pin-cushion distortionof the rasters resulting from curvature of the screen.

With the foregoing arrangement, however, the following undesirableeffect is produced due to the fact that the horizontal deflection fieldis of the pin-cushion-type configuration. When the three beams B_(R),B_(G) and B_(B) originating in a common horizontal plane arehorizontally deflected to scan screen S at the opposite side portion ofthe latter, rasters L_(R), L_(G) and L_(B) resulting from the "red,""green," and "blue" beams B_(R), B_(G) and B_(B) should be located atequal distances from each other. However, as shown in full lines on FIG.3, beams B_(R), B_(G) and B_(B), when deflected horizontally to one orthe other of the side portions of screen S, do not land on the latter atequally spaced locations with the landing spot of beam B_(G) beingmidway between the landing spots of beams B_(R) and B_(G). The foregoingtendency results from the fact that, with the horizontal deflectionfield having a pin-cushion-type configuration to compensate forspherical aberration as described above, the side beam that originatesat the side of the central beam B_(G) in the direction of horizontaldeflection, for example, the side beam B_(B) as shown on FIG. 3, passesfor a relatively short distance through a strong portion of thehorizontal deflection field, whereas the other side beam B_(R) passesfor a large distance through a weak portion of the horizontal deflectionfield. Thus, the beams B_(B) and B_(R) are deflected through anglessufficient to cause their convergence at a common point on thebeam-selecting grid or mask G_(P). However, the central beam B_(G),which also passes through a relatively weak portion of the horizontaldeflection field, travels through such portion of the field for adistance that is shorter than the path of beam B_(R) in the horizontaldeflection field and, as a result thereof, the deflection of beam B_(G)is less than that required to cause proper convergence of the latterbeam with beams B_(B) and B_(R). In other words, when side beams B_(B)and B_(R) properly converge at a common point on one or the other of theside portions of grid or mask G_(P), the central beam B_(G) arrives atthe grid or mask at a point that is displaced inwardly from that commonpoint, as shown in full lines on FIG. 3.

In accordance with this invention, the above undesirable effect ordeviation is corrected by shielding the side beams B_(B) and B_(R) froma leakage flux produced by the horizontal deflection field of yoke 20,while permitting such leakage flux to act fully on central beam B_(G) sothat the latter will be horizontally deflected by a greater angle tocrossbeams B_(R) and B_(B) at the common point of convergence of thelatter at grid or mask G_(P), as indicated in broken lines at B'_(G) onFIG. 3. When the invention is applied to a color cathode-ray tube of thetype shown on FIG. 1, the shielding of side beams B_(R) and B_(B) fromthe leakage flux from the horizontal deflection field may beadvantageously effective at the exit of such beams from the convergencedeflecting means F.

More specifically, as shown on FIGS. 4 and 5 such convergence deflectingmeans F may have its electrode plates P and P' attached to the endsurface of cylindrical grid G₅ through conductive angle members 51 and51' respectively. Electrode plates Q and Q' are attached to insulatingmembers 53 and 53' mounted on support pins 52 and 52' extending from theelectrode plates P and P' respectively. Further, a brush or coil springmember 55 is secured to a bracing member 54 bridging the free ends ofelectrode plates P and P' so as to maintain a spacing between theseelectrode plates. Member 55 is in electrical contact with a conductivelayer 56 extending over the inner surface of the neck portion N, and towhich an anode voltage V_(P) is applied by way of an anode button (notshown). Hence, such anode voltage is applied to electrode plates P andP'. Plates Q and Q' are connected with each other through a conductorwire 57, and a conductor wire 59 extends from electrode plate Q forexample to a button 58 provided in the neck portion N for example, sothat a voltage that is 200 to 300 volts lower than anode voltage V_(P)can be thereby applied to electrode plates Q and Q'.

In accordance with this invention, magnetic shielding members Y and Y'are mounted on the outer surfaces of electrode plates Q and Q',respectively, adjacent the ends of the latter remote from electrode G₅.Each of these magnetic shielding members Y and Y' may include a flat orstraight portion 60 (FIG. 5) extending across the correspondingelectrode plate Q or Q', and bent end portions 61 and 62 which extendinwardly from the opposite ends of straight portion 60.

With such an arrangement, magnetic leakage flux from the horizontaldeflection field produced by deflection yoke means 20 and extending overthe lateral regions between the plate P and Q and plates P' and Q' canpass through the opposing magnetic shielding members Y and Y', asindicated by the arrows 63 on FIG. 6. It is obvious that because of thedescribed configuration of magnetic shielding member Y and Y', thehorizontal field distribution density in the portion of the spacetherebetween through which center beam B_(G) passes is higher than thehorizontal field distribution densities in the portions of such spacethrough which side beams B_(R) and B_(B) pass, as shown in FIG. 6. If itis assumed that the horizontal deflection magnetic leakage flux entersmagnetic shielding member Y at its end portion 61, it will be apparentthat the leakage flux occurring over the lateral extent of such endportion 61 will be collected thereby and the magnetic flux thuscollected will arrive at end portion 62 of shielding member Y, and thenbe expanded. Similarly, the horizontal deflection magnetic leakage fluxenters shielding member Y' at its end portion 61 which collects theleakage flux over its lateral extent, and the flux thus collectedtravels through straight portion 60 to arrive at end portion 62 ofshielding member Y' and then be expanded. On the other hand, thehorizontal deflection leakage flux located laterally between the endportions 61 and the end portions 62 of shielding members Y and Y' aresubstantially uninhibited by the latter and thus act on center beamB_(G) to relatively increase the horizontal deflection of the latter andthus restore the center beam to the position shown at B'_(G) on FIG. 3.

Since shielding members Y and Y' are located at the exit end ofconvergence deflecting means F, that is, close to thehorizontal-vertical deflection yoke 20, it will be apparent that theleakage flux from the horizontal deflection field of yoke 20 which isallowed to act only on the center beam B_(G) will then be of relativelyhigh density to provide a sufficiently large corrective or additionaldeflection to center beam B_(G) for correcting the deviation shown onFIG. 3. On the other hand, if shielding members Y and Y' were disposedat the entry end of convergence deflection means F, that is, adjacent toelectrode G₅, the leakage flux of the horizontal deflection field wouldthere be of insufficient density to provide the requisite additionaldeflection to center beam B_(G) when allowed to act on that beam whilebeing excluded from acting on side beams B_(R) and B_(B).

It will be apparent that the shielding members Y and Y', being disposedoutwardly with respect to the paths of side beams B_(R) and B_(B), willalso collect the leakage flux, indicated in broken lines at V on FIG. 6,from the vertical deflection field of yoke 20, whereby the density ofthe vertical deflection leakage flux will tend to be greater between theinwardly directed ends 61 and the inwardly directed ends 62 of shieldingmembers Y and Y', and a reduced density of vertical deflection leakageflux is present between straight portions 60 of the shielding members.However, since all three beams B_(R), B_(G) and B_(B) are locatedbetween straight portions 60, the vertical deflection leakage fluxexisting between the latter will not have any substantial differentialeffect with respect to the vertical deflection of the beams. This is tobe distinguished from the arrangement disclosed in copending U.S.application Ser. No. 796,838, filed Feb. 5, 1969, which is now U.S. Pat.3,548,249, granted Dec. 15, 1970, and having a common assignee herewith,and in which magnetic yoke members are mounted on the plates P and P' atthe entry to convergence deflection means F so as to respectively extendbetween center beam B_(G) and side beam B_(R) and between the centerbeam and the other side beam B_(B). Such magnetic yoke members have bentend portions which extend outwardly therefrom and which serve to collectthe leakage flux from the vertical deflection field of yoke 20 so thatthe leakage flux of the vertical deflection field occurring outside oneof the magnetic yoke members is collected thereby and passes, with arelatively high density, to the other magnetic yoke member from whichthe flux again expands. The result of the foregoing is that theconcentration of the leakage flux between the yoke members, that is, inthe space through which the central beam passes provides a differentialvertical deflection of the central beam with respect to the side beamsso as to avoid a vertical deviation of the center beam from the point ofconvergence of the side beams when such beams are vertically deflectedtoward the top or bottom of the screen. Although the described yokemembers do provide the side beams with some shielding from the leakageflux of the horizontal deflection field, such leakage flux is too smallat the entry to the convergence deflection means F to provide therelatively large additional deflection or correction required forcorrecting the deviation of the center beam shown on FIG. 3 hereof.Further, if the yoke members of U.S. application Ser. No. 796,838 werelocated at the exit end of convergence deflecting means F, whereby toprovide a significant additional horizontal deflection or correction tothe center beam, the increased density of the leakage flux from thevertical deflection field at such location would result in an excessivevertical deflection or correction being imparted to the center beam.Thus, in correcting for both vertical and horizontal deviations, it isdesirable to provide the yoke members of U.S. application Ser. No.796,838 at the entry to convergence deflecting means F and the shieldingmembers Y and Y' of this application at the exit from the convergencedeflecting means.

The above-described horizontal corrective effect can be produced merelyby providing the magnetic shielding members Y and Y' at the exit fromconvergence means F. Furthermore, the magnetic correcting field neededfor assisting the horizontal deflection of the center beam is thenobtained from the leakage component of the horizontal deflection fieldproduced by deflection yoke means 20. Therefore, there is no need toprovide any special electromagnetic means to produce the deflectioncorrecting field.

.[.Although the leakage component of the horizontal deflection field isused for aiding the horizontal deflection of the center beam in thedescribed embodiment, it will be apparent that it is also possible toproduce the aforementioned effect by providing additional, externalelectromagnet means or permanent magnet means to produce magnetic fluxacting on the center beam and from which the side beams are shielded bymember Y and Y'..].

In the above description of the invention, it has been assumed that theshielding members Y and Y' according thereto are employed to correct adeviation of the raster of center beam B_(G) from the rasters of sidebeams B_(R) and B_(B) that may remain even when the horizontal andvertical deflection coils are given the configuration described withreference to FIGS. 2A and 2B. However, it will be appreciated that, whenthe horizontal and vertical deflection coils are not given theconfiguration of FIGS. 2A and 2B, the shielding members Y and Y' can bestill employed to correct the aforementioned deviation between therasters of the three electron beams.

In the described embodiment of the invention, the three beams originatein a horizontal plane. However, these three beams may originate in avertical plane, in which case the horizontal deflection coils should bewound in a saddlelike form to produce a barrel-shaped horizontaldeflection field, and the vertical deflection coils should be wound in atoroidal form to produce a pin-cushion-shaped field. Furthermore, insuch a case, the direction in which the phosphor stripes of the screenand the grid wires of the grid extend should be changed to thehorizontal direction, and the convergence deflecting means F is turnedthrough 90° to effect convergence of side beams B_(B) and B_(R)vertically toward center beam B_(G).

.[.Further, the application of the present invention is not limited totubes in which the plural beams originate on a straight horizontal orvertical line. Thus, the invention is also applicable to tubes in whichthe origins of the three beams are in a delta arrangement, and the threebeams are intended to converge at a common point on the beam-selectinggrid or mask adjacent the color screen. In this case too, the presentinvention can be employed to increase the horizontal deflection of thecenter beam relative to the deflections of the side beams..].

Having described a particular embodiment of the invention with referenceto the accompanying drawings, it will be understood that the inventionis not limited to such precise embodiment, and that various changes andmodifications, only some of which have been mentioned above, may be madetherein without departing from the scope or spirit of the invention.

What we claim is:
 1. In a color cathode-ray tube having means generatingplural beams .Iadd.including a central beam and opposite side beamswhich originate in a common horizontal plane and .Iaddend.which aredirected, at predetermined incident angles to each other for convergenceon a screen, through horizontal and vertical deflection fields producedby electromagnetic deflection means and by which said beams are made toscan said screen, the improvement comprising magnetic shielding meansdisposed .Iadd.in advance of said electromagnetic deflection means.Iaddend.adjacent the paths of said beams through said deflection fieldsand being operative to .[.selectively.]. .Iadd.equally .Iaddend.shield.[.at least one of.]. said .Iadd.opposite side .Iaddend.beams fromleakage flux from said horizontal deflection field while permitting saidleakage flux to act on .[.another of said beams.]. .Iadd.said centralbeam .Iaddend.for correcting deviations between the rasters of saidplural beams on said screen. .[.2. A color cathode-ray tube according toclaim 1, in which said plural beams originate in a common plane andinclude a central beam and opposite side beams, and in which saidmagnetic shielding means are positioned to permit only said central beamto be acted upon by said leakage flux while shielding said side beamstherefrom..].
 3. A color cathode-ray tube according to claim .[.2.]..Iadd.1.Iaddend., in which said magnetic shielding means includes twospaced-apart shielding members disposed at outer sides of the paths ofsaid side beams and being shaped to collect and to direct around saidside beams the leakage flux that would otherwise act on said side beams.4. A color cathode-ray tube according to claim 3, in which each of saidshielding members includes a substantially straight portion extendingsubstantially parallel to and in spaced relation to the straight portionof the other shielding member, and end portions at the ends of saidstraight portion and being directed at substantial angles to the latterin the direction toward the corresponding end portions of the other ofsaid shielding members.
 5. A color cathode-ray tube according to claim4, in which .[.said common plane is horizontal and.]. said straightportions of the yoke members extend substantially vertically so thatsaid shielding members collect and direct around said side beams theleakage flux from the horizontal deflection field in the lateral regionsthrough which said side beams pass.
 6. In a single-gun, plural-beamcolor picture tube which includes a color screen having arrays of colorphosphors and beam-selecting means provided with apertures correspondingto said arrays, beam-generating means for directing a central electronbeam and two side electron beams in a common plane toward said screenfor impingement on respective phosphors of each array through thecorresponding aperture, lens means for focusing said electron beams onsaid screen and having an optical center at which said beams are made tocross each other with said side beams emerging from said lens meansalong paths lying in said plane and which are divergent with respect tothe central beam, electron beam convergence deflecting means operative,upon the application of a convergence deflecting voltage thereto, todeflect said side beams emerging along said divergent paths forconvergence of all of said beams at an aperture of said beam-selectingmeans, and deflection yoke means having sweep signals applied thereto toprovide fields which deflect said beams in directions respectivelyparallel and at right angles to said plane for causing said beams toscan said screen; the improvement comprising magnetic shielding meansdisposed .Iadd.in advance of said deflection yoke means.Iaddend.adjacent the exit from said convergence deflecting means andshielding only said side beams from leakage flux from said field whichdeflects said beams in said direction parallel to said plane whilepermitting said leakage flux to pass unimpeded in a zone through whichsaid central beam passes for correcting deviations in the direction ofsaid plane between the positions of the rasters on the color screenproduced by said beams in scanning said screen.
 7. A single-gun,plural-beam color picture tube according to claim 6, in which saidmagnetic shielding means includes spaced-apart shielding membersarranged at the outer sides of said side beams and each having asubstantially straight portion at right angles to said plane and endportions at the ends of said straight portion and being directedinwardly at substantial angles to the latter.